Method for forming a glass-to-metal seal



Jan. 26, 1965 E. C. HOPKINS ETAL METHOD FOR FORMING A GLASS-TO-METAL SEAL Filed April 5, 1961 QTTCQNEY S United States Patent Office 3,167,418 Patented Jan. 26, 1955 3,167,418 METHOD FDR FORMING A GLASS-T- METAL SEAL Ethan C. Hopkins and Clifford W. Ask, South Woodstock,

Conn, assignors to American Optical Company, Southbridge, Mass, a voluntary association of Massachusetts Filed Apr. 5, 1961, Ser. No. 101,621 2 Claims. (Cl. 6543) The field of this invention is that of hermetic sealing techniques, and the invention relates, more particularly, to novel and improved methods for hermetically sealing windows or other optical elements within metallic supporting frames.

In hermetically sealing windows and the like within metallic frames or mountings, a selected fusing medium is conventionally heated in situ between prepared surfaces of the window and frame elements, the selected fusing medium being adapted to fuse to materials embodied in each of the elements. The temperature to which the fusing medium is heated for effecting such a seal frequently must be limited on the one hand to avoid softening or sagging of the window element and on the other hand to avoid excessive oxidation of the metallic frame material such as would impair the quality of the bond formed between the fusing medium and the frame. It may be that wetting of the Window element by the fusing medium would be enhanced at relatively high fusing temperatures for achieving a better seal between the fusing medium and the window element but that the described seal might be formed at a relatively low fusing temperature to avoid excessive oxidizing of the frame material. On the other hand, it may be that a relatively low fusing temperature might be employed to avoid softening or sagging of the window element even though the fusing medium could be more securely fused to the metallic frame at a relatively higher fusing temperature. Thus the temperature at which such a seal is effected generally comprises a compromise between preferred temperatures at which the fusing medium could be adhered to the window and frame elements individually. The need for such a compromise requires that the selected fusing medium be adapted to fuse to vastly different materials at a single fusing temperature and seriously limits the materials which can be used for forming hermetically sealed units by conventional methods as will be understood. Further, it is frequently inconvenient to maintain a window element and a fusing medium in proper positional relation within a metallic frame during formation of a hermetic seal by conventional techniques.

It is an object of this invention to provide a novel and improved method for forming a hermetic seal between two elements by means of a fusing medium; to provide such a method for forming hermetic seals wherein a fusing medium is fused to each of said elements individually under different environmental conditions; to provide such a method for hermetically sealing a dielectric material suchas glass, sapphire or the like to a metallic material; to provide such an improved method for hermetically sealing a relatively thin window or optical element of a dielectric material within a metallic frame or mounting whereby the temperature at which the window is fused to said fusing medium can be substantially increased; and to provide such a method for forming hermetic seals which can be conveniently and economically performed.

Briefly described, the method provided by this invention for forming a hermetic seal between two elements such as a Window of dielectric material and a metallic window window frame with a selected fusing medium includes the step of initially coating one of the elements such as the window with the fusing medium by applying the fusing medium to the element in melted form at a temperaturc at which the fusing medium is adapted to wet and to be fused to the element. In one aspect of this invention, where a relatively thin window of dielectric material having relatively broad window surfaces is to be hermetically sealed within a peripheral metallic frame, the window element is preferably provided in the form of a rod which has a cross-sectional configuration corresponding to the configuration of the window surfaces of the desired window element. A selected fusing medium is then heated to a fusing temperature such that the fusing medium has a maximum viscosity of approximately 10 poises but does not tend to volatilize or otherwise decompose and such that the rod can be immersed therein for a period long enough to achieve wetting of the rod by the medium while permitting portions of the rod to remain below the Littleton softening temperature of the rod material in supporting relation to remaining portions of the rod. The rod is immersed in said fusing medium for forming a fused coating of the fusing medium thereon and is then removed from the medium before deformation of the rod occurs. Since the rod has a substantial volume which will heat to a deforming temperature relatively slowly, and since the rod can be rapidly coated with a fusing medium which hasheen preheated to fusing temperature, the fusing medium can be fused to the rod at a relatively high temperature without deforming the rod. The coated rod is then cooled to substantially room temperature and is cut transversely of its longitudinal axis for forming at least one window element having a highly-adherent, peripheral coating of the fusing medium thereon. I

The coated window element is then disposed within the metallic frame element with said window element coating substantially in contact with the frame, and the frame element is heated for heating the window element coating adjacent the frame to a fusing temperature by heat transfer from the frame element, this heating being rapidly accomplished for achieving said fusing temperature at the junction of the frame and fusing medium coating before any significant part of the window element itself is elevated to a similar temperature, thereby to fuse the window element coating to said metallic frame element without heat-softening the Window element Other objects, advantages and details of the method provided by this invention will appear in the following detailed description of a preferred embodiment of the invention, the detailed description referring to the drawings in which:

FIG. 1 is a perspective view of a rod of dielectric window material to be hermetically sealed within a metallic mounting; i

FIG. 2 is a perspective view of a metallic window mounting;

FIG. 3 is a diagrammatic section view illustrating a step in preparing the window material according to the process of this invention;

FIG. 4 is a partial side elevation view of the rod of window material illustrating a subsequent step in preparation of the window material;

FIG. 5 is an enlarged sectional view of a window element prepared according to the method of this invention taken as on line 55 of FIG. 4; and p FIG. 6 is an enlarged diagrammatic section view illustrating the final step provided by this invention for hermetically sealing a prepared window element within the metallic mounting.

Referring to the drawings, 10 in FIG. 1 illustrates a rod of glass or other similar dielectric material of the type from which a window or related optical element is to be formed to be hermetically sealed Within a metallic mounting, the rod having a cross-sectional size and configuration corresponding to the configuration of a window or cilitating adhesion of the rod to a selected fusing medium.

A metallic frame or ring 14 can be provided to serve as a supporting frame for an element to be formed from the rod 10, the frame having an aperture 16 which is of a size such that the frame can be fitted over the rod 10. Preferably, also, the frame has an inner surface 13 of a shape generally conforming to the peripheral configuration of the rod '10. The frame can be flanged asshown at 20 in FIG. 2 or can have any other outer configuration within the scope of this invention. The inner surface 18 of the frame'can also be polished or cleaned or otherwise treated in any conventional manner for facilitating adhesion of the rod to a selected fusing medium.

holder to form a hermetic seal between the frame 14 and a window formed from the rod by means of a fusing medium according to this invention, the fusing medium can be selected in view of certain conventional criteria as well as in view of certain factors which will be explained below. Thus in effecting a hermetic seal between a selected fusing medium and a metallic frame, the junction of the frame and fusing medium must be elevated to a predetermined temperature for inducing a preferred degree of oxidation of the frame and for melting the fusing medium so that the medium is adapted to wet the oxidized frame. Selection of such a fusing medium and determination of such a fusing temperature for forming a hermetic seal to a metallic element can be readi-" 1y accomplished by the well known techniques which are presently employed for forming conventional glass-tometal seals. The same techniques can be employed for selecting a fusing medium and for determining a fusing temperature for adhering said medium to a metallic frame according to the method of this invention. For example, where the frame 14 embodies a conventional stainless steel material, a solder glass may preferably be hermeticallysealed to the stainless steel frame either by conventional method or by the method of this invention at a fusing temperature of approximately 1100" F.'for inducing a preferred degree of oxidation of the frame material; Accordingly, a conventional lead borate glass having a melting temperature of approximately 650 F. can be utilized as the solder glass for assuring that the solder glass can be sufliciently melted to properly, wet and be fused to the oxidized frame at 1100 F. Of course, many other similar solder glasses or fusing mediums could also be selected in the same manner to be fused to the metallic frame at similar fusing temperatures.

Similarly, in effecting a hermetic seal between the se lected fusing medium and a window or the like of dielectricmaterial, the junction of the window element and the fusing medium must be elevated to a predeterminable temperature for adapting the fusing medium to wet the higher temperature up to temperatures at which the fusing medium would volatilize or otherwise decompose :ould be also utilized, generally with enhanced wetting of I the window material by the fusing medium, These wetting temperature ranges are well known or can be readily determined by conventional techniques for various fusing mediums which might be employed to form hermetic seals with the various materials which can be used for forming windows and other optical elements. However, it is also generally true, particularly where the window element is relatively thin and has relatively broad window surfaces, that the window element will tend to soften or sag or 7 otherwise deform to anexcessive extent at temperatures substantially lower than a temperature which might be preferred for hermetically sealing a fusing medium to the window material.

According to this invention, a selected fusing medium is preferably-placed in a suitable receptacle 22 and is heated, as is diagrammatically indicated by the heating coil 24 in FIG. 3, so that the fusing medium 26 is reduced to a molten state at a selected temperature within the wetting temperature range determined as above described for the dielectric material embodied in the rod 10. The rod 10, supported by any suitable hanger means 28, is then immersed in the fusing medium for a very short period of. time to coat the rod 10 with a layer of the fusing medium of predetermined thickness. Immersion coating of the rod 10 to form the coated rod 10a in this manner introduces two factors relating to the selected temperature at which the fusing medium should be maintained during immersion of the rod. First, the fusing medium is preferably heated to a sufliciently high temperature for reducing the viscosity of the fusing medium below a maximum of approximately 10 poises, whereby the fusing medium will have flow characteristics such that the medium is adapted to form a relatively'thin and uniform coating on the'rod. The minimum temperature to which the fusing medium must be heated for displaying the noted viscosity is well known or can be readily determined by conventional techniques for various ma-.

terials which might be employed as fusing mediums. Second, the fusing medium must be maintained at a temperature which is sufficiently low so that the rod can be immersed in the fusing medium for a period of time sufficient to achieve wetting of the rod surface by the medium while maintaining portions of the rod below the Littleton softening temperature of the rod material in supporting relation to remaining portions of the rod. As will be understood, immersion of the rod 10 in the heated fusing medium will first heat the skin or peripheral portions of the rod to a temperature approaching the tem perature of the fusing medium and will also rapidly heat the rod inwardly toward the center of the rod. Whether the peripheral portions of the rod will be elevated to the desired wetting temperature before the central portions of the rod will be heat-softened to such an extent that they will no longer support the rod without deformation depends upon such factors as the temperature of the fusing medium, the thermal inertia of the rod material and the Littleton softening temperature of the rod material but also depends to a large extent upon the physical configuration of the rod itself. Thus the maximum temperature at which the fusing medium can be maintained to permit immersion coating of the rod therein without suggested which is adapted to display a maximum viscos-- ity of approximately 10 poises at a temperature above; 1400 F. and which does not tend to volatilize to a sig-- nificant extent or to otherwise decompose at a temperature. below 1800 F. The rod,10 can be approximately /3. inch in diameter and preferably is of a length substantially greater than its diameter. The rod can embody aconventional glass having a strain point of approximately 650 F. and a Littleton softening tempera-ture of approximately 1000 F., for example. Such a rod' can be immersed. in the fusing medium while the medium is maintained at a temperature as high as 1700 F. for providing the rod with a coating of the fusing medium which is hermetically sealed to the rod without softening central portions of the rod to such an extent that the rod is deformed.

During immersion of the rod in the fusing medium for a period. of about seconds a coating of approximately 0.015 inch thickness can be formed on the rod. Preferably the rod is heated to a temperature above the strain point of the rod material prior to immersion in the molten fusing medium, about 650 F. in the above example, to assure that the rod can be immersed in the fusing medium without tending to shatter or craze. Of course, where a relatively thick coating of the fusing medium is to be formed :in the rod, the rod canbe repeatedly im: mersed in the fusing medium for building up a coating of the desired thickness, the rodbeing permitted to cool between said immersions as will be understood.

After removing the rod 10 from immersion in the fusing medium 26, the coated rod is slowly cooled in a well known manner for preventing the establishment of internal stresses within the rod. The rod need not be cooled completely to room temperature but should be substantially cooled to assure solidity of the rod and coating and preferably [0 permit convenient handling of the rod during subsequent operations according to the method of this invention. The rod is then cut transversely of its longitudinal axis by any conventional cutting means such as the diamond-tipped cutting wheel 30 which is diagrammatically indicated in FIG. 4, thereby to form a coated window element 32 as shown in FIG. 5. As shown, the window element 32 will thus embody a central window portion 32a of dielectric rod material and a highly adherent, peripheral coating 26a of the selected fusing medium. The window element 32 can be polished or otherwise treated in any conventional manner for improving the optical qualities of the window portion 32a of the element, and, if desired, the window coating 26a can be ground or otherwise treated for adjusting the diameter or thickness of the coating. It should be noted that the coated rod 10 can be cut to form more than one window element and can also be cut to provide optical elements such as lenses and the like in well known manner, the peripheral portions of such lenses also having a coating of the fusing medium. It should also be understood that, although the window element 32, as herein described, is coated in the form ofa rod and is then cut to form a window element, a single window element could be coated by immersion in a heated fusing medium 26 as above described. However, as will be understood, a single window element, particularly a window element which is relatively thin, can be more rapidly heated to a deforming temperature so that the maximum temperature at which said single window element could be coated in the manner described would be substantially lower than the temperature at which the same material could be coated in rod form. The rod or a single window element could also be coated with a molten fusing medium by other means such as spray coating within the scope of this invention.

The window element 32 is then disposed within the metallic frame '14 as shown in FIG. 6 and is preferably supported in concentric relation therein by a supporting block 33. The supporting block is preferably formed of a suitable refractory material such as is disclosed in US. Patents Nos. 2,440,187 or 2,764,491 which does not tend to stick to molten glass. Of course, the coating 26a formed on the window element 32 must be of predetermined thickness so that the window element will fit closely within the frame aperture 16 with the window element coating approximately in contact with the inner frame surface 18 as will be understood. The window element 32 and the frame 14 are then supported within an induction heating coil 34 as is diagrammatically illustrated in FIG. 6. The induction heating coil 34 is adapted to heat the metallic frame 14 internally in well known manner without tending to radiate heat or otherwise generate heat which would heat the window element 32 directly. Such heating can be rapidly accomplished as is well known so that the portion of the window element coating 26a which is approximately in contact with the metallic frame 14 will also be rapidly heated by conduction from the metallic frame. According to this invention, this heating of the metallic frame should be accomplished so rapidly that the junction of the frame and the coating of fusing medium 215a is elevated to the predetermined preferred fusing temperature for sealing the coating to the frame before heat conducted through the coating can elevate the window portion 32a of the window element to a deforming temperature. Preferably the junction of the frame and the coating 26a is elevated to said fusingtemperature before the junction of the coating 26a and the window portion 3-211 is elevated to a wetting temperature. Where the window element coating 26a is relatively thin, any melted portion of the coating 26a will tend to be retained at the junction between the coating and the metallic frame by capillary action as will be understood. For example, where the frame 14 and the window element 32a are formed of materials as suggested in the above example, and where a window element of inch thickness and /3 inch diameter having a coating 26a of 0.015 inch thickness is disposed within the frame aperture 16 with approximately 0.002 maximum clearance, the frame can be inductively heated as above described within 45 seconds for establishing a preferred fusing temperature of approximately 1100 F. at the junction of the frame and the coating 2 6a without tending to excessively soften or sag the window portion 32a. This is a preferred temperature for achieving adequate wetting of the frame 14 by the fusing medium embodied in the coating 26a for securing a highly adherent, bubble-free .bond between the frame and coating as noted above. It should be understood that this invention is not limited to the use of inductive heating means as described above, but includes any means for heating the frame 14 which is also adapted to rapidly heat the window element coating Zea primarily by means of heat transfer from the frame.

It should also be understood that although a particular embodiment of the method of this invention has been described for the purpose of illustration, this invention includes all modifications and equivalents thereof which fall within the scope of the appended claims.

Having described our invention, we claim:

1. A method for hermetically sealing a relatively thin window of dielectric material having relatively broad window surfaces within a peripheral metallic frame by means of a fusing medium, said method comprising the steps of providing a rod of dielectric material having a cross-sectional configuration corresponding to the configuration of said window surfaces, heating said fusing medium to a predetermined temperature, said temperature being sufiiciently high so that the medium is adapted to fuse to said rod material and has a maximum viscosity of 10 poises, said temperature being sufiiciently low for preventing substantial volatilization of the medium and so that the rod can be immersed therein for fusing a coating of the medium thereon while permitting portions of the rod to remain below the Littleton softening temperature of the rod material in supporting relation to other portions of the rod, immersing said rod in said fusing medium for fusing a coating of the medium thereon, removing the coated rod from said medium before deformation of the rod occurs, substantially cooling the rod, cutting the rod transversely of its longitudinal axis for forming a relatively thin window element having a peripheral coating of said fusing medium thereon, disposing the coated window element within said metallic frame with said peripheral coating approximately engaging the frame, and heating that portion of the frame for heating said coating approximately engaging the frame to a predetermined fusing temperature by heat transfer from the frame for fusing the frame to said coating before the window element is heated to a deforming temperature.

2. A method for hermetically sealing a relatively thin window of dielectric material having relatively broad winenema-1 etional' configuration corresponding to the configuration of said Window surfaces, heating said rod above the strain point temperature of the rod material, heating said fusing medium to a predetermined temperature, said temperature being suificiently high so that the medium is adapted to fuse to said rod material and has a maximum viscosity of 10 poises, said temperature being sufliciently low for preventing substantial volatiliz-ation of the medium and so thatthe rod can :be immersed therein for fusing a coating of the medium thereon while permitting portions of the rod to remain below' the Littleton softening temperature of the rod material in supporting relation to other portions of the rod, immersing said rod in said fusing medium for fusing a coating of the medium thereon, removing the coated rod from said medium before the rod is deformed, substantiallycooling thearod, cutting the 'rod transversely. of its longitudinal axis-for forming: arelatively thin window element having a peripheral coating of said' fusing medium thereon, disposing the coated window element within said metallic frame with said peripheral coating approximately engaging the frame, and inductivelyheating the frame for heatingsaid coating to a predetermined fusing temperature. by heat transfer from the frame for fusing the frame to said eoatingbefore the window element is heated to:a deforming temperature,

References Citedinthe file of this patent: I UNITED STATES PATENTS Henry et all Tune 14, 1958 

1. A METHOD FOR HERMETICALLY SEALING A RELATIVELY THIN WINDOW OF DIELECTRIC MATERIAL HAVING RELATIVELY BROAD WINDOW SURFACES WITHIN A PERIPHERAL METALLIC FRAME BY MEANS OF A FUSING MEDIUM, SAID METHOD COMPRISING THE STEPS OF PROVIDING A ROD OF DIELECTRIC MATERIAL HAVING A CROSS-SECTIONAL CONFIGURATION CORRESPONDING TO THE CONFIGURATION OF SAID WINDOW SURFACES, HEATING SAID FUSING MEDIUM TO A PREDETERMINED TEMPERATURE, SAID TEMPERATURE BEING SUFFICIENTLY HIGH SO THAT THE MEDIUM IS ADAPTED TO FUSE TO SAID ROD MATERIAL AND HAS A MAXIMUM VISCOSITY OF 10**4 POISES, SAID TEMPERATURE BEING SUFFICIENTLY LOW TO PREVENTING SUBSTANTIAL VOLATILIZATION OF THE MEDIUM AND SO THAT THE ROD CAN BE IMMERSED THEREIN FOR FUSING A COATING OF THE MEDIUM THEREON WHILE PERMITTING PORTIONS OF THE ROD TO REMAIN BELOW THE LITTLETON SOFTENING TEPERATURE OF THE ROD MATERIAL IN SUPPORTING RELATION TO OTHER PORTIONS OF THE ROD, IMMERSING SAID ROD IN SAID FUSING MEDIUM FOR FUSING A COATING OF THE MEDIUM THEREON, REMOVING THE COATED ROD FROM SAID MEDIUM BEFORE DEFORMATION OF THE ROD OCCURS, SUBSTANTIALLY COOLING THE ROD, CUTTING THE ROD TRANSVERSELY OF ITS LONGITUDINAL AXIS FOR FORMING A RELATIVELY THIN WINDOW ELEMENT HAVING A PERIPHERAL COATING OF SAID FUSING MEDIUM THEREON, DISPOSING THE COATED WINDOW ELEMENT WITHIN SAID METALLIC FRAME WITH SAID PERIPHERAL COATING APPROXIMATELY ENGAGING THE FRAME, AND HEATING THAT PORTION OF THE FRAME FOR HEATING SAID COATING APPROXIMATELY ENGAGING THE FRAME TO A PREDETERMINED FUSING TEMPERATURE BY HEAT TRANSFER FROM THE FRAME FOR FUSING THE FRAME TO SAID COATING BEFORE THE WINDOW ELEMENT IS HEATED TO A DEFORMING TEMPERATURE. 